The present invention relates to a honeycomb filter used in a burner (e.g. a boiler) or an exhaust gas purifier of an internal combustion engine (e.g. a diesel engine), as well as to a process for producing such a honeycomb filter.
Recently, the influence of a particulate matter emitted from a diesel engine, etc., on environment has been taken up seriously. As an important means for capturing and removing such a particulate matter, a honeycomb filter for exhaust gas purification is in use.
The honeycomb filter for exhaust gas purification generally has a structure possessing a plurality of through-holes which are separated from each other by porous partition walls and which are plugged alternately at its exhaust gas inlet face and its exhaust gas outlet face. The honeycomb filter captures and removes a particulate matter contained in an exhaust gas, by forcibly allowing an exhaust gas which enters thereinto from the open through-holes of the exhaust gas inlet face, to pass through the partition walls in the filter.
By the way, such a honeycomb filter is, for its feature, exposed to the rapid temperature change of exhaust gas or locally heated to a high temperature, during the use; the temperature distribution of the filter becomes nonuniform; as a result, the thermal stress of the filter increases due to the difference in the thermal expansion among the respective portions constituting the filter, and consequently the filter generates cracks, etc. This problem is being pointed out.
When the honeycomb filter is used particularly as a means for exhaust gas purification in diesel engine, etc., the filter is locally heated to a high temperature in the filter regeneration step in which the fine carbon particles deposited on the filter is burnt for removal; therefore, generation of cracks, etc. in the filter is being taken up as a serious problem.
Further, since such a filter is exposed to very high temperatures, there is being used a filter composed mainly of highly heat-resistant silicon carbide, or of metallic silicon and silicon carbide. However, silicon carbide, etc. have a drawback of high thermal expansion coefficient; therefore, the filter generates a large thermal stress and has had a problem of generating cracks, etc. more easily.
For these problems, there have heretofore been proposed various honeycomb structures each obtained by bonding a plurality of honeycomb segments via a bonding material in order to reduce the thermal stresses of various positions of honeycomb structure (these positions are restrained by each other and are unable to deform freely under the thermal stresses).
In, for example, JP-B-61-51240 is disclosed a thermal shock-resistant rotary heat regenerator obtained by bonding a plurality of ceramic honeycomb segments with a bonding material having substantially the same mineral composition as the honeycomb segment and showing a thermal expansion coefficient different from that of the honeycomb segment by 0.1% or less at 800xc2x0 C.
In this rotary heat regenerator, however, since the mineral composition of the bonding material is substantially the same as that of the honeycomb segment, the individual honeycomb segments are greatly restrained by the bonding material and there has been a problem in that the thermal stress generated by each honeycomb segment cannot be reduced sufficiently.
Also, in JP-A-8-28246 is disclosed a ceramic honeycomb filter improved in the durability of the basal body of honeycomb filter, obtained by bonding a plurality of honeycomb segments with an elastic sealing material formed by bonding at least a three-dimensionally interwoven inorganic fiber and inorganic particles with each other via an inorganic binder and an organic binder.
In this honeycomb filter, however, since the thermal conductivity of the sealing material is low and the individual honeycomb segments are thermally shielded from each other, the temperature distribution among the respective portions constituting the filter cannot be uniformed and the reduction in the thermal stress generated in the honeycomb filter has not been fully sufficient.
The present invention has been made in view of the above-mentioned problems, and aims at providing a honeycomb filter which can reduce the thermal stresses among the respective portions constituting the filter and can highly suppress generation of cracks, etc. and which functions well particularly when composed mainly of silicon carbide or of metallic silicon and silicon carbide, and a process for producing such a honeycomb filter.
The present inventor made a study in order to achieve the above aim. As a result, the present inventor found out that the above aim can be achieved by securing a high thermal conductivity in the whole portion of a honeycomb filter to be produced with contacting each two adjacent honeycomb segments each other at each portion of the sides facing each other, and further by employing such a structure that honeycomb segments are bonded at least at part of each portion other than the above portion (this portion is hereinafter referred to as xe2x80x9ccontact portionxe2x80x9d in some cases), directly or via a heat-conductive member, via a bonding material having a strength lower than that of a basal body material of honeycomb segment. The present invention has been completed based on the above finding.
According to the present invention, there is provided a honeycomb filter obtained by bonding a plurality of honeycomb segments each having a plurality of through-holes which are separated from each other by porous partition walls and which are plugged alternately at the exhaust gas inlet face and exhaust gas outlet face of honeycomb segment,
characterized in that each two adjacent honeycomb segments are allowed to contact with each other at each portion of their sides facing each other and are bonded with each other at least at part of each portion of said sides other than the contacted portion via a bonding material having a strength lower than that of a basal body of honeycomb segment.
In the present invention, it is preferred that the each portion of two sides of each two adjacent honeycomb segments facing each other, other than the portion at which they are contacted with each other directly or via a heat-conductive member (the former portion is hereinafter referred to as xe2x80x9cnoncontact portionsxe2x80x9d in some cases) is provided so as to include at least each whole part of the sides having the contact portion, opening to the exhaust gas inlet face of honeycomb segment. More preferably, the portion is provided so as to include at least each whole part of the sides having the contact portion, opening to the exhaust gas inlet face of honeycomb segment, the exhaust gas outlet face of honeycomb segment and an outer surface of honeycomb filter. In this case, it is also preferred that the noncontact portion includes at least each part of said sides having the contact portion, leading to the exhaust gas inlet face and the bonding material is provided at part of the non-contacted portion.
In the present invention, it is also preferred to have such a structure that at least one of the each two adjacent honeycomb segments has a projection at a portion of its side facing one side of the other honeycomb segment and contacts, via this projection, with a portion of one side of the other adjacent honeycomb segment, or that at least one of the each two adjacent honeycomb segments has a heat-conductive member composed of the same material as that of the honeycomb segments at a portion of its side facing one side of the other honeycomb segment and contacts, via this heat-conductive member, with a portion of the side of the other adjacent honeycomb segment.
In the present invention, it is also preferred that each honeycomb segment is composed mainly of silicon carbide or of metallic silicon and silicon carbide, that the bonding material is composed mainly of a heat-resistant inorganic compound, and that the partition walls formed between the through-holes of honeycomb segment have a metal having a catalytic activity, loaded thereon.
Meanwhile, according to the present invention, there is provided a process for producing a honeycomb filter,
characterized by
kneading a powder material composed of a ceramic and/or a metal, a binder and water to obtain a puddle and molding the puddle to obtain a honeycomb structure having a plurality of through-holes separated by partition walls from each other,
drying the honeycomb structure and then plugging the plurality of through-holes alternately at the exhaust gas inlet face and exhaust gas outlet face of honeycomb structure to obtain a dried honeycomb segment,
bonding, to a portion of each side of the dried honeycomb segment, a heat-conductive member composed of the same material as that of the dried honeycomb segment, followed by drying and firing to obtain a composite honeycomb segment,
contacting the heat-conductive member-provided honeycomb segment, via the heat-conductive member, with a portion of a side of other adjacent honeycomb segment facing the former honeycomb segment, and providing, at least at part of a noncontact portion, a bonding material having a strength lower than the strength of a basal body of honeycomb segment, to obtain a unified body.
According to the present invention, there is also provided a process for producing a honeycomb filter,
characterized by
kneading a powder material composed of a ceramic and/or a metal, a binder and water to obtain a puddle and molding the puddle to obtain a honeycomb structure having a plurality of through-holes separated by partition walls from each other,
drying the honeycomb structure and then plugging the plurality of through-holes alternately at the exhaust gas inlet face and exhaust gas outlet face of honeycomb structure to obtain a dried honeycomb segment,
subjecting the dried honeycomb segment to a cutting operation to form a projection at each side of the dried honeycomb segment, and conducting firing to obtain a honeycomb segment,
contacting each projection formed at each side of the honeycomb segment, with a portion of a side of other adjacent honeycomb segment facing the former honeycomb segment, and providing, at least at part of a noncontact portion, a bonding material having a strength lower than the strength of the dried honeycomb segment, to obtain a unified body.
In these processes of the present invention, the powder material is preferably composed mainly of silicon carbide or of metallic silicon and silicon carbide. Also, the bonding material is preferably composed mainly of a heat-resistant inorganic compound.
Thus, in the present invention, since each two honeycomb segments are bonded with each other at least at each portion with a bonding material having a strength lower than that of the basal body of honeycomb segment, the restraint of each honeycomb segment by the bonding material is low and the individual portions of filter can be freely deformed corresponding to their changes in temperature. Consequently, the thermal stress generating in filter is low even when nonuniform temperature distribution appears in various portions of the filter during the use of the filter; as a result, generation of cracks can be prevented and a honeycomb filter superior in durability can be obtained.
Further, in the present invention, since a plurality of segments are in contact with each other at each portion of their sides facing each other, the contact ensures the thermal conductivity of whole filter together with a bonding material or with no help of such a bonding material and the temperature distribution in filter can be uniformized. As a result, the thermal stresses in various positions of filter during the use of filter can be further reduced, in addition to the above-mentioned effect obtained by the bonding material, and generation of cracks can be prevented considerably. Such an effect is particularly striking when the honeycomb segments are composed mainly of highly heat-conductive silicon carbide, or of metallic silicon and silicon carbide.